1. Field of the Invention
The present invention relates to a skin treatment composition such as a cosmetic composition and a pharmaceutical preparation. More specifically, it relates to a skin treatment composition having a remarkably improved skin depigmentation effect and is very safe to use, as well as suppressing skin irritation caused by an ultraviolet (UV) absorber when the UV absorber is contained therein.
2. Description of the Related Art
Although the mechanisms of pigmentation such as melasma, chloasma, and the like in the skin are not fully understood, it is believed that a melanin pigment is formed and abnormally deposited in the skin due to hormone abnormalities and UV light irritation caused by sunlight. The above-mentioned pigmentation is generally treated by administering substances suppressing the formation of melanin. For example, vitamin C is administered in a large amount; glutathione or a similar compound is injected; or L-ascorbic acid, cysteine, or a similar compound is applied to localized areas in the form of an ointment, cream, or lotion.
In the United States and Europe, hydroquinone preparations are used as pharmaceuticals. These substances, however, suffer from various disadvantages. For example, L-ascorbic acid and its derivatives of higher fatty acids are disadvantageous in the stability thereof. In particular, when they are used in the presence of water, they cause unpreferable discoloration and an unpleasant odor. Thiol compounds such as glutathione and cysteine have irritant or unpleasant odors and, in addition, are susceptible to oxidation. Therefore, these compounds are generally not incorporated into skin treatment compositions such as cosmetic compositions.
Furthermore, the effects of the above-mentioned compounds for the intended treatment appear too weak, and the depigmentation effects are not satisfactory. On the other hand, although the depigmentation effects of hydroquinone on the skin are apparently recognized, the use of hydroquinone in skin treatment compositions is generally limited due to its contact allergenicity. Various attempts have been made to improve the disadvantage of hydroquinone and make it safer to use. For instance, the use of the mono esters of higher fatty acids of hydroquinone has been proposed to solve this problem. These esters, however, are easily hydrolyzed in the presence of esterases in vivo, and thus these esters are not necessarily safe to use.
Furthermore, conventional skin treatment compositions often contain UV absorbers to prevent the localized areas from sunlight, because melanization is promoted by the irradiation of sunlight. Nevertheless, the skin irritation due to the application of UV absorbers still occurs in some cases, depending on some type of UV absorbers used and individual differences in skin properties.